#include<iostream>
using namespace std;

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode() : val(0), left(nullptr), right(nullptr) {}
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
    TreeNode(int x, TreeNode* left, TreeNode* right) : val(x), left(left), right(right) {}
    
};

class Solution {
public:
    long ret = LONG_MIN;
    bool isValidBST(TreeNode* root) {
        return dfs(root);
    }

    bool dfs(TreeNode* root)
    {
        if (root == nullptr)
        {
            return true;
        }
        bool left = dfs(root->left);
        if (left == false) return false;
        bool cur = false;

        if (root->val > ret)
        {
            cur = true;
        }

        if (cur == false) return false;
        ret = root->val;
        bool right = dfs(root->right);
        return left && right && cur;

    }
};